1. Field of the Invention
The present invention relates to an apparatus for detecting the presence of a liquid. In particular, the present invention relates to an apparatus having an electrically passive sensor adapted for placement in a region where the presence of a liquid is to be detected, and an electrically active detector that draws information from the sensor through mutual inductance or conductivity when activated by the user.
2. Discussion of Background
Liquid-presence and liquid-level detection are important in a wide variety of situations, both in the industrial process environment and elsewhere. Many industrial liquids are hazardous to individuals and to the environment. In such situations, prompt detection and repair of leaks is imperative. Leaks in concealed or buried structures--including structures such as wall, floor and ceiling-mounted piping, underground pipelines and storage tanks, between liners of landfills, outside swimming pool liners, dams, roofs, foundation walls, attic-mounted or roof-mounted heating and air conditioning systems--may go undetected for extended periods of time. The longer a leak is undetected, the more likely it is to cause structural or environmental damage, and the more expensive it is to repair.
A common approach to leak and spill prevention is the use of secondary containment systems, where a second barrier surrounds the main liquid container. On a laboratory scale, this may be as simple as carrying or storing a reagent bottle in a bucket. If the bottle is cracked, or if its cap is not tightly sealed and the contents leak, the liquid is held inside the bucket where--at least in principle--it will do no damage and will be promptly noticed and dealt with. Many storage tanks and process pipelines have double walls so that, if the inner wall leaks, the outer wall contains the liquid. However, it is often difficult to detect leaks in the inner wall before they cause more extensive structural damage.
Existing liquid-level and liquid-presence detectors respond to the electrical and thermal conductivity, capacitance, density, surface tension, and the chemical, optical and acoustic properties of the liquid of interest. For example, Bryan, Jr., et al. (U.S. Pat. No. 4,598,273) and Farris, et al. (U.S. Pat. No. 4,502,044) show moisture detection systems for roofs. The Bryan, Jr. system comprises a plurality of independent and insular moisture detecting units placed in a predetermined spatial relationship across a surface, such as a roof structure, and beneath an otherwise water-impermeable membrane. Each detecting unit includes moisture detection, power supply, and signaling units. As water penetrates holes in the membrane of the surface, the water activates a battery that supplies power to the signaling unit. Farris, et al. show a grid having at least two conductors with a plurality of spaced-apart extensions. The grid provides an impedance between the conductors that decreases in the presence of moisture. An audible signal generating circuit is coupled to the detector and responds to the resultant decrease in the impedance. Purtell, et al. (U.S. Pat. No. 4,246,575) discloses a moisture-activated switch comprising a compressed dehydrated cellulose sponge wafer having conductive plates attached to opposing faces. When the wafer swells in response to contact with moisture, a bridging conductor electrically connects the two plates.
None of these devices is widely applicable to detecting leaks in secondary containment systems, where the sensing must typically be done in very close quarters, often in the gap between two closely-spaced walls. In addition, an environment of use may be corrosive, and a relatively large amount of liquid, identical with that whose leakage is to be detected, is almost always located close by, just on the other side of the inner or primary containment. Furthermore, any breach in the outer, secondary barrier--for the penetration of wiring or optical fibers--is undesirable since it, too, could act as a path for leakage.
Prompt detection of fluids is also important from the standpoint of comfort and sanitation. For example, infants and many incontinent adults use diapers, which must be changed at intervals. An infant may be aware that a diaper is wet and needs changing, but might not communicate that need effectively. Some adults may sense when a diaper is wet, however, others may find it difficult or impossible to do so. Regardless of whether traditional cloth diapers or leak-resistant disposable diapers are used, there is at present no convenient means--short of opening or removing the diaper--of determining when it needs to be changed. Resealing a previously-sealed disposable diaper is difficult because the waterproof cover may be turn when its sealing tapes are opened, thus, even a dry diaper must usually be discarded after opening. A safe, convenient and inexpensive device for checking diaper wetness in situ, without requiring opening or removal of the diaper, would result in increased comfort for users and prevent the premature disposal of large numbers of diapers.
Conductivity probes are available for detecting the presence of moisture between two spaced-apart electrodes, however, these could cause shock or injury when used with humans. Okada, et al. (U.S. Pat. Nos. 4,754,264 and 4,653,491) disclose capacitance-detecting devices that sense the degree of wetness of a diaper and transmit the information to a remote location. Both devices use a sensing unit composed of a water impermeable upper sheet, a water permeable lower sheet, and a pair of metal layers placed between the sheets in parallel. One of the metal layers is covered with an electrical insulating layer. The presence of water is detected through the change in electrostatic capacitance between the two metal layers. These devices may be uncomfortable for long-term use, since, like typical conductivity probes, they depend on conductive wires to transmit a signal to a remote sensing unit. In addition, both devices are continuously activated so that the monitor can respond to the presence of a liquid when the diaper is wetted. There is no presently-available, electrically passive device that may be inserted inside a diaper at intervals to check on its state (i.e., wet or dry), or left in situ with no possibility of current flow that might injure the wearer.
There remains a need for an apparatus for sensing the presence of a liquid that would be usable with virtually any liquid regardless of its density, electrical or optical properties, that would be unaffected by ambient conditions such as temperature, and that would sense moisture without the need for physical contact between the detector and the sensor, or between the detector and the region wherein a liquid is to be detected.